In order to deal with future depletion of energy, campaigns for promoting energy saving and for recycling resources as well as development of techniques for accomplishing these purposes are spreading. Especially in recent years, it has been strongly desired, as a global effort, to reduce CO2 emission resulting from fuel combustion in order to prevent global warming.
An example of an internal combustion engine with less CO2 emission is a diesel engine used for motor vehicles and the like. Even with less CO2 emission, however, the diesel engine suffers from a problem of black smoke discharged from the engine. Black smoke is produced when oxygen runs short with respect to the injected fuel. Specifically, a part of fuel thermally decomposed causes a dehydrogenation reaction that produces a precursor of black smoke, and this precursor is thermally decomposed again and agglomerated and joined to produce black smoke. The black smoke produced in this manner causes air pollution, and may have an adverse influence on human bodies.
The amount of the black smoke generated can be reduced by raising the fuel injection pressure of fuel injected into the combustion chamber of diesel engine. For this purpose, however, the steel pipe used for fuel injection is required to have a high fatigue strength. As a manufacturing method for the steel pipe used for fuel injection, the following invention has been disclosed.
Patent Document 1 (JP9-57329A) discloses a manufacturing method for a steel pipe used for fuel injection of a diesel engine, in which the inner surface of hot rolled seamless steel pipe stock is ground by shot blasting, and then cold drawing is performed.
This Document describes that by using this manufacturing method, the depth of a flaw (irregularities, fracture, microcrack, and the like) on the inner surface of steel pipe can be not deeper than 0.10 mm, so that a high strength of the steel pipe used for fuel injection can be attained.
Although it has a high strength, the steel pipe used for fuel injection manufactured by the method disclosed in Patent Document 1 cannot have a fatigue life corresponding to the strength of the steel pipe material. Naturally, as the strength of the steel pipe material increases, the pressure applied to the inside of the steel pipe can be increased; a limiting internal pressure (hereinafter, referred to as a “critical internal pressure”), within which a fracture due to fatigue does not occur on the inner surface of steel pipe when a pressure is applied to the inside of the steel pipe, does not depend only on the strength of steel pipe material. That is to say, even if the strength of steel pipe material is increased, a critical internal pressure cannot be obtained as expected. Considering the reliability or the like of final product, a longer fatigue life is desirable; as the critical internal pressure is reduced, however, the fatigue life is also reduced because the steel pipe is prone to fatigue through the use under high internal pressure.
In particular, the recent tendency aims at higher internal pressure; specifically, the steel pipe is required to have a tensile strength of not lower than 900 N/mm2, and a corresponding improvement in fatigue characteristics is desired.